Which leads to an obvious question: How do they determine when they have laid the right number? To find out, scientists experimented by going to nests and repeatedly removing eggs soon after they were laid. Some birds replaced them straight away. For example, a House Sparrow laid 50 in a row, while a flicker laid 71 in 73 days before the removal was halted.

Birds that do this — keep on laying after eggs are removed — are called indeterminate layers. And for them, it seems that the number of eggs is intimately tied to the brood patch. That's the bare skin on a bird’s underside that transfers body warmth to the eggs during incubation. It's the tactile sensation involved in this process that seems to let the layer know she's reached the ideal number of eggs.

But for some other birds, the scientists’ removing eggs had no effect at all on egg numbers. These are known as determinate layers. And just why these two types of layers exist remains an open question.

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